This invention relates to optical signal processing and transmission in fiber-optic communication systems.
Fiber-optic communication systems require transmitters that can encode optical signals with digital information. The two most widely used data formats are non-return-to-zero (NRZ) where the intensity does not go to zero between continuous 1 bits and return-to-zero (RZ) where the intensity returns to zero between continuous 1s as shown in FIG. 1. Optical transmitters using external high-speed modulators with low-chirp are essential for high-data-rate long-haul communication systems. Lithium niobate electro-optic intensity modulators have nearly zero output optical frequency modulation or chirp and are commonly used in commercial transmitters. However, they require large driving voltages at high bit rates (&gt;2.5 Gb/s). Semiconductor electroabsorption modulators are key components for high-speed transmitters because they can be driven at low voltages (&lt;3 Volts) even at high bit rates as described by Kataoka et al. in Electron. Lett., vol. 28, pp. 897-898 (1992). They can be constructed to be polarization insensitive, and are integrable with semiconductor lasers and amplifiers. However, they have two drawbacks: limited modulation speed caused by device capacitance and chirp.
There are two main types of electroabsorption modulators, namely, multiple-quantum-well and bulk waveguide devices. Both of these modulators operate under the principle that the optical absorption coefficient of the semiconductor increases in response to an applied electric field as described by Wood in J. Lightwave Technology, vol. 6, p. 743 (1988) and Keldysh in Soviet Phys. JETP, vol. 34, p. 788 (1958). This effect is intrinsically high-speed with response time in the sub-picosecond range. However, the modulation bandwidth of electroabsorption modulators is critically limited by device capacitance. The capacitance can be reduced somewhat by decreasing device area and/or increasing the waveguide layer thickness. But this comes at a significant trade-off of lower modulation on/off ratio, higher drive voltages, and more complicated and expensive device packaging.
Intensity modulators with low or negative chirp parameters are essential for long-haul transmission in conventional 1.3-.mu.m zero-dispersion optical fiber. The chirp parameter is a measure of the broadening of the spectrum of the modulated optical signal relative to the amount of its amplitude modulation as described by Devaux et al., in J. Lightwave Technology, vol. 11, p. 1937 (1993). The chirp parameter of electroabsorption modulator varies between -5 to 5 depending on many factors such as material structure, input wavelength, and applied voltage. This should be compared with the Mach-Zehnder type electro-optic intensity modulator with a chirp parameter approaching zero independent of applied voltage as described by Koyama et al. in "Frequency chirping in external modulators," J. Lightwave Technology, vol. 6, p. 87 (1988). Nonetheless, propagation in conventional fibers up to 300 km at 2.5 Gb/s using electroabsorption modulator-based transmitter has been demonstrated by Ishimura et al. in "Small chirp and wide bandwidth integrated modulator-laser at zero offset-bias operation," ECOC '97 Technical Digest, p. 171, 1997. At higher bit rates, the maximum propagation distance is limited because of the finite output chirp from the electroabsorption modulator. Chirp leads to severe signal distortion, inter-symbol interference, and even timing jitter after propagation through long distance of dispersive optical fiber. This results in unacceptable bit-error-rate at the receiver.
Electroabsorption modulators exhibit a highly nonlinear transfer characteristic versus voltage as described by Devaux et al. in "Optical processing with electroabsorption modulators," OFC '98 Technical Digest, p. 285, 1998. This nonlinear transfer characteristic is a result of the exponential nature of the absorption edge of the semiconductor. The nonlinearity can be exploited in applications such as all-optical wavelength conversion as described by Edagawa et al. in OFC '97 Technical Digest, p. 77, 1997, and generation of optical pulses as described by Devaux in "Light pulse generator," U.S. Pat. No. 5,559,628. The speed of the electroabsorption modulator for these applications depends on how well the modulator can respond to high-speed electric field modulation and is also limited by the device capacitance. Because of the finite size of the device, there is a lower limit of the capacitance and thus there exists an ultimate limit of the modulation bandwidth of electroabsorption modulators as described by Wood. Beyond this limit, optical signals from the modulator are severely distorted.